The present invention is in the field of manufacturing plates of hard-to-work metals, such as chromium and its alloys, by powder-metallurgy.
The most common metal-working or fabrication processes for producing plate-like products are rolling and forging. In both these processes, a thick metal preform formed by casting or other methods is shaped into a relatively thin product by plastic deformation under the influence of applied stresses. The metal elements and alloys in use today exhibit a very wide range of work-ability characteristics. Some of these, such as copper, aluminum, and mild steel, are very easy to work, whereas others, such as chromium, molybdenum, tungsten, and their alloys, and some complex superalloys, are very difficult to fabricate. Processes such as open-die forging and rolling generate tensile stresses leading to tensile deformation and reduction in thickness. If the metal or alloy being worked or deformed is not sufficiently ductile under the working conditions, it will tend to fracture or develop surface or end cracks. With hard-to-work alloys, fracturing and cracking account for the greatest material loss in the fabrication process.
Open specific use of a metal plate is as a target in a sputtering apparatus, such as that described by John S. Chapin, "The Planar Magnetron", Research/Development, Vol. 25, No. 1, pp 37-40 (January 1974). In a sputtering process, an anode and a cathode or target comprising a layer of metal to be sputtered are placed in a chamber containing an ionizable gas at a reduced pressure. When the electrodes are connected to a source of electric potential, metal is removed from the target and deposited as a thin film on a substrate disposed nearby.
The currently available methods for preparing large-area sputtering targets of chromium, its alloys, and similar hard-to-work materials have distinct disadvantages. One such method involves casting plates of the sputtering material, machining them to the proper shape, and mounting one or more in an array to obtain a target of the desired size. This method is disadvantageous for the reasons that castings of chromium and the like are quite brittle, thus being very difficult to machine and tending to crack under high temperature gradients which arise during sputtering at high powers. Such cracking can cause leakage of circulating water which is used to cool the cathode during the sputtering process. Similarly, brazing or other bonding of a rolled sheet of the sputtering metal to a backing plate is disadvantageous because of the difficulty in working such materials without developing cracks and fractures. Another method involves electroplating chromium onto a backing plate. This is disadvantageous because it is exceedingly difficult to electroplate many metal elements and most alloys.
Many of the difficulties associated with the normal casting procedures for producing chromium and chromium-alloy plates can be overcome by a powder-metallurgy technique such as described by R. W. Fountain, author of Chapter 7, "Chromium", at page 106 of Rare Metals Handbook, 2nd Edition (1961), edited by Clifford A. Hampel, and published by Reinhold Publishing Corporation. Briefly, this technique involves cold-compacting the powder, usually with a binder, at pressures of 40,000 to 60,000 lb/in.sup.2, and vacuum-sintering in two steps at temperatures of 2400.degree. F. (1315.degree. C.) and above. Typically, the product has a relatively low density, such as less than 90% of the theoretical maximum. The process requires specialized equipment, and is limited to products of relatively small areas because of the pressure required in the compacting step.
The pressure required to consolidate powder can be greatly reduced by pressing the powder while it is hot. In a paper, "Fabrication of Beryllium Sheet from Hot Pressed Powder", which appeared at pp 5-11 of Vol. 17 of "Progress in Powder Metallurgy" (Metal Powder Industries Federation 1961), B. H. Hessler and J. P. Denny describe a technique for forming billets by hot-pressing beryllium powder in a mild steel die. A billet is then machined to form a slab, encapsulated in mild steel for protection and restraint, and rolled to form a sheet of the desired thickness.